Equalization consists in linearly filtering the transmitted signal in order to obtain a transmission path whose overall response is flat in amplitude and linear in phase even though the path makes use of transmission cables whose attenuation is proportional to cable length and to the square root of the transmitted signal frequency, with different coefficients depending on the structures of the cables.
Equalization is automatic if the linear filter used for equalization purposes is provided with variable components which are adjusted by a servo-control system responsive to modifications in the characteristic of the transmitted signal so as to adapt the gain transfer characteristic of the filter to the real attenuation of the section of cable used, said adaptation generally being an adaptation in length, i.e. a compensation for variation in the attenuation characteristic of a cable of a kind that can be considered as being due to variations in the length of the cable.
Most prior automatic equalizers in use for base band data transmission are adjusted by means of a peak amplitude detector, thereby providing automatic gain control. Examples are described in French Pat. Nos. 2 128 152 and 1 603 582 (equivalent U.S. Pat. No. 3,568,100). These circuits are designed for use with coaxial cables of constant structure along their entire length, and their performance is greatly reduced when used in the public switched telephone network (PSTN) where a link can be made up from several successive lengths of pairs having different gauges.
One attempt at solving this problem seeks to make use of a criterion which is more representative of variations in the attenuation characteristic of a cable as a function of frequency, such criterion is based on the regularity of the zero crossings in the transmitted digital signal. An implementation is described in French Pat. No. 2 419 618 (equivalent U.S. Pat. No. 4,243,956) making use of a sign coincidence autocorrelator which operates on two mutually delayed versions of the transmitted and equalized digital signal. This new criterion considerably improves the quality of the equalization, but suffers from the drawback of requiring, in its preferred implementation, a pre-distortion filter placed at the input to the sign coincidence autocorrelator, thereby limiting the range over which the automatic equalizer can adapt automatically.
The object of the present invention is to provide an automatic equalizer for synchronous digital transmission, said equalizer having a wide range over which it can adapt automatically and being usable in the PSTN.